We propose to study the nature and genetic control of immune responses to cells infected with viruses and other antigens in inflammatory bowel disease. Lymphocytotoxicity tests will be conducted using lymphocyte effectors from patients and controls and infected and non-infected intestinal target cells. Relevant populations of cytotoxic cells will be identified. The possibility that cytotoxic reactions are determined by HLA phenotypes will be explored. The genetics of IBD will be studied by typing a population of patients for serologically and lymphocyte-defined HLA antigens. Correlations will be sought between markers of susceptibility and the segregation of the major histocompatability complex in IBD families. The interrelationships between various antigens, and immunological and genetic factors in IBD will be analyzed.